108-convert-sorted-array-to-binary-search-tree

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {number[]} nums
 * @return {TreeNode}
 */
var sortedArrayToBST = function(nums) {
    let helper = function(start, end){
        if(start > end){
            return null;
        }

        const mid = Math.floor( (start+end) / 2);
        const root = new TreeNode(nums[mid]);
        root.left = helper(start, mid-1);
        root.right = helper(mid+1,end);
        return root;
    }

    return helper(0,nums.length-1);
};